510 zyxwvutsrqpon Inorganic Chemistry, Vol. 18, No. zyxwvutsr 2, zyxwvutsrq 1979 Sharrock, Thibaudeau, and CaillE Contribution from the Dipartement de Chimie and the Dipartement de Physique, Universiti de Sherbrooke et Groupe de Recherche sur les Semiconducteurs et les Diilectriques, Sherbrooke, Province de Quebec, Canada J1K 2R1 Magnetic Studies of Two New Copper Hippurate Dimers P. SHARROCK,* C. H. THIBAUDEAU, and A. CAILLB Received May 22, 1978 The EPR spectra and temperature-dependent magnetic susceptibility of two copper(I1)-amino acid complexes have been determined. Results suggest a dimer structure for both the dimethyl sulfoxide and the dimethylformamide adducts of zy bis(N-benzoylglycinato)copper(II). The temperature variation of the magnetic susceptibility of these compounds shows a broad maximum near 265 K, indicating a strong antiferromagnetic interaction between the Cu2+ ions within the pairs. A superexchange mechanism via the bridging carboxylate groups is compatible with our results and suggests that N-substituted amino acids, or small peptides, can act as simple carboxylate ligands for copper(II), providing model complexes for the EPR-nondetectable copper in enzymes. Introduction One of the characteristic reactions of amino acids is the formation of blue copper chelates.' Peptides also bind copper in a chelatelike fashion, deprotonated peptides linkages forming stable Cu-N bonds.2 N-Benzoylglycine, hippuric acid, is unusual in that its substituted nitrogen does not seem to coordinate to copper, while its acidic function acts as a bridging bidentate group. A considerable amount of research has been reported on the magnetic properties of polynuclear copper(I1) complexe~.~-~ Two different types of binuclear complexes have been found with a variety of carboxylate ligands. A first type of com- pounds, of which copper acetate monohydrate is the simplest example, is characterized by strong exchange interaction between copper ions of spin zyxwvutsrq 1/2 forming isolated pairs. The antiferromagnetic exchange interaction results in a S = 0 singlet ground state and S = 1 triplet state. The separation between these states is 2 J = -285 cm-] in copper acetate monohydrate,6 and qualitative evidence points to a superex- change mechanism of interaction occurring via the bridging carboxylate groups and not via a direct metal-metal bond.7 On the other hand, monoatomic oxygen bridges, such as found in the copper hippurate tetrahydrate complex,s are present in another class of compounds characterized by smaller 2Jvalues ranging from -18 to -40 Recently, several groups of workers have studied the copper-hippuric acid ~ystem,I'-'~ and very low temperatures were needed to detect the weak antiferromagnetic interaction in the tetrahydrate, for which -2J = 4.3 cm-' (or 6.2 K). The ammine adducts seem to be monomeric. We report EPR and magnetic susceptibility results for the first adducts of a copper-amino acid complex found to exhibit strong antiferromagnetic interactions. Experimental Section Synthesis. N-Benzoylglycine (hippuric acid, Hip) was purchased from British Drug House Biochemicals, and the anhydrous copper complex was prepared by standard methods.15 The adducts Cu- (Hip),-MezSO and Cu(Hip),.DMF were isolated by dissolving an- hydrous copper hippurate in pure Me2S0 or DMF and by evaporating the excess solvent under vacuum. Quantitative analysis by Chemalytics Inc., Tempe, Ariz., gave satisfactory results for the MezSO adduct. Anal. Calcd for CU(C~H~CONHCH~COO)~.(~H~)~SO: C, 48.23; H, 4.45; N, 5.62. Found: C, 48.1 1; H, 4.45; N, 5.63. However, only approximate results for the less air-stable DMF adduct were obtained. Anal. Calcd for Cu(C6H5CONHCHzC00)2.(CH3)2NCHO: C, 51.14; H, 4.70; N, 8.53. Found: C, 50.17; H, 4.44; N, 8.29. EPR Spectra. EPR spectra were recorded at X-band frequencies on a Varian E-9 spectrometer at liquid nitrogen and room tem- peratures. The magnetic field was swept from 0 to over 8 kG and calibrated every 1 kG with a digital N M R gaussmeter. Room- temperature K-band spectra were obtained with a JESME ESR *To whom correspondence should be addressed at the DCpartement de Chimie. 0020-1669/79/1318-0510$01 .OO/O Table I. Magnetic Parameters for Copper Hippurates at 77 K and X-Band Frequencies anhydrous Me,SO adduct DMF adduct g I1 2.363 2.363 2.353 gl 2.071 2.075 2.065 F 2.146 2.149 2.139 DF cm-' 0.37 0.35 0.34, D, cm-' 0.352 0.345 0.342 2 J, cm- 327 292 i 14 313 r 17 a Approximate values derived from the difference between the perpendicular components of room-temperature 24.8-GHz and 35.0-GHz spectra. E, cm-' <0.01 <0.01 <0.01 spectrometer. Several samples of each complex were run and showed identical features. Static Magnetic Susceptibility. The magnetic susceptibility of finely powdered samples has been measured in a Faraday apparatusI6 over the temperature range 65-320 K. The apparatus was calibrated with a 99.999% pure platinum sample, using 201.9 X 10" emu/mol as the value of its susceptibility at room temperature. Measurements at different field strengths, up to 8 kG, showed no dependence of the susceptibility on the applied field. The overall relative accuracy of the measurements was found to be 1%. Results EPR Spectra. Figures 1-3 show the first-derivative X-band EPR spectra of the anhydrous compound and the Me2S0 and the DMF adducts of Cu(Hip),. In each case, an absorption is observed near 3000 G which arises from mononuclear impurities of spin S = The intensity of this absorption increases as the temperature is lowered. Calculation of the corresponding anisotropic g factors gives gll = 2.36 and g, = 2.07 in the three cases examined.17 The amount of impurities appears to be especially important in the less stable DMF adduct. The other absorption lives are due to transitions in the triplet state.18 Both Cu(Hip),.Me2S0 and CU(H~P)~.DMF show well-defined hyperfine structure, in their low-temperature spectra, whereas the anhydrous compound does not. These transitions may be represented by a S = 1 spin Hamilfonian of the form in eq 1, where p is the Bohr magneton, H is an H = @figs + D(SZ2 - y3) + E(S2 - Sj2) (1) external-magnetic field, g is an anisotropic Land&splitting tensor, S is the total spin vector, and S,, S,, and S, are the spin operators of its mutually perpendicular components. The four transitions resolved in the 77 K spectra allowed us to calculate the zero-field splitting parameters D and E and to obtain values for g and g, following the procedure de- scribed by Chasteen.14! Results are shown in Table I. Magnetic Susceptibility. Diamagnetic corrections were determined by the method of Pasca120 (-221.7 X 10" emu/mol 0 1979 American Chemical Society